The present invention relates generally to waveguide lenses and, more particularly, to waveguide lenses characterized by wide bandwidth and polarization insensitivity.
Waveguide lenses are used to focus electromagnetic energy to or from a feed, or a cluster of feeds. Such a lens generally comprises an assemblage of short waveguide elements positioned side by side in a two-dimensional array, with the combined inner and outer surfaces shaped generally (but not necessarily) to a lens contour, although in a zoned waveguide lens there may be physical step discontinuities between zones. Several varieties of waveguide lenses exist. The zoned variety of waveguide lens is made of hollow waveguides, and its outer surface is stepped in concentric rings of appropriate radii. Other varieties employ various forms of phase shifters in the waveguide elements to produce the phase correction required for focussing.
The design of such lenses where wide bandwidth is desired involved a number of dilemmas, discussed in detail hereinafter. As brief examples of such dilemmas, the constant group delay lens contour surfaces and the constant phase delay lens contour surfaces do not coincide. Physical zoning steps of some designs introduce polarization-sensitive variations, serious phase rotations, and shadowing.
By way of example; the following U.S. patents are identified as disclosing various known forms of waveguide lenses: Kock U.S. Pat. No. 2,562,277; Kock U.S. Pat. No. 2,576,463; Kock U.S. Pat. No. 2,596,251; Kock U.S. Pat. No. 2,599,763; Kock U.S. Pat. No. 2,603,749; Affel, Jr. U.S. Pat. No. 2,607,009; Kock U.S. Pat. No. 2,640,154; Kock U.S. Pat. No. 2,712,067; Crawford U.S. Pat. No. 2,729,816; Kock U.S. Pat. No. 2,733,438; Rust et al U.S. Pat. No. 2,764,757; Kock U.S. Pat. No. 2,769,171; Proctor, Jr. U.S. Pat. No. 2,834,962; Young, Jr. U.S. Pat. No. 2,841,793; Berkowitz U.S. Pat. No. 3,049,708; Dion U.S. Pat. No. 4,156,878; and Coulbourn, Jr. U.S. Pat. No. 4,194,209. Further examples are disclosed in the literature, such as Dion and Ricardi, "A Variable-Coverage Satellite Antenna System," Proc. IEEE, Feb. 1971, pp. 252-262.
Somewhat related to waveguide lenses are dielectric lenses, representative examples of which are disclosed in the following U.S. Patents: McMillian U.S. Pat. No. 2,985,880; Cary et al U.S. Pat. No. 3,886,558; and Beyer U.S. Pat. No. 3,886,561. Other related lenses are disclosed in Cohn U.S. Pat. No. 2,617,936 and Kock U.S. Pat. No. 2,747,184.
As discussed in detail hereinafter, an important characteristic of waveguide lenses in accordance with the invention is wide bandwidth without physical zoning steps, and without sensitivity to polarization. Accordingly, it is relevant to consider specifically several of the patents identified above which employ zoning techniques to achieve a wider bandwidth, but avoid physical zoning steps.
As one specific example, the disclosure of the Proctor, Jr. U.S. Pat. No. 2,834,962 points out that physical zoning steps can be avoided by providing a variable refractive index lens wherein different waveguides of the lens have different refractive indices at a particular frequency. Proctor, Jr. further discloses the provision of two separately-proportioned, differently-loaded regions in each waveguide having different phase velocities and different refractive indices. Proctor, Jr. describes the use of ridging along on in conjunction with a dielectric material for varying the cut-off frequency of the waveguide channels. However, all of the approaches which Proctor, Jr. illustrates are suitable only for a single linear polarization, and not for orthogonal linear polarizations (and hence, not for circular polarization). Furthermore, they involve two distinct sections (focussing and compensating), or sometimes three along the waveguide axis, where the length of each section may vary as a function of its transverse location.
As another specific example, the Dion U.S. Pat. No. 4,156,878 discloses smooth lens contour surfaces between which there are separate delay and phase compensating sections. The Dion lens is zoned in the sense that it employs pins, the rotational angle of which should periodically return to the starting point as radius increases. Due to the nature of the phase shifting mechanism, the Dion lens is fundamentally limited to a single (generally circular) polarization (e.g., right circular but not left). Thus, it is not suitable for polarization diversity. (It is believed to actually reverse the rotation of a circularly polarized wave passing through.) Further, the Dion lens appears restricted as a practical matter to the use of circular waveguides in order to allow mechanical rotation of the phase shifters.
By the present invention there are provided relatively thin waveguide lenses characterized by wide bandwidth, improved phase length match for all ray paths through the lens across the lens without physical zoning steps, and polarization insensitivity.